It has been observed that there is an increase in overall resistance when multiple contacts are formed in a series of constructions such as in a continuing chain of connections, in a stack of contacts on top of each other, and/or a combination thereof. It has also been confirmed that the increase in resistance is related mainly to large interfacial contact resistance. For example, aluminum oxide surfaces or films formed on top of aluminum contact structure have been most notably demonstrated to exacerbate this behavior of causing increased resistance in the overall metal structure formed thereby.
In order to combat and/or mitigate this increase in resistance, several approaches have been tested and/or developed. For example, one typical approach adopts to remove the oxide film at the interfacial surface and to make direct contact to the surface of bulk metal underneath. However as is known in the art, even though this approach may remove the oxidized surface film, it nevertheless does not address the likelihood of re-oxidation of the metal surface underneath before the completion of the process of forming the contact. Even during in-situ removal of surface contaminants such as metal-oxide in a vacuum system, residual atmosphere may still promote formation of undesired films. On the other hand, an ex-situ cleaning process may suffer from poor uniformity of an oxidized surface layer, and as such the varying depth of oxidative surface products may require excessive cleaning, which in turn may be destructive to other structures such as dielectric surfaces during a non-selective excessive clean step that is required to assure a clean surface.